Are you 100% sure she was not communicating with a Supervisor or Manager? If as shoppers we are waiting in a lengthy queue and staff appear to open other till points, it is often because a till operator has alerted a Supervisor/Manager there are queues. Till operators can also have someone on the shop floor relaying prices/barcodes to them if something won't scan - also saving time for those queuing.
Having had a career in retail/merchandising I can also tell you that one of the biggest gripes till operators have is the increasing number of customers coming through their tills while on their mobile phone, delaying everyone in the queue by using only one hand to pack/expecting the till operator to do it all for them, throwing their shopping bags at the till operator, and worst of all - not acknowledging the staff member in any fashion whatsoever.